1. Field of the Invention
The present invention relates to a method for the preparation of a diamond film-coated body. More particularly, the present invention relates to a method for the preparation of a diamond film-coated body so unlikely to cause flaking and so high in durability as to be suitable for cutting tools and so on. The method encompasses forming a diamond film in a substantially uniform film on a plane surface of a body having a corner portion, such as a cutting tool having a cutting part, and on a portion in a ridge of the corner portion of the body and in the vicinity of the portion of the ridge as well.
2. Description of Related Art
Recently, cutting tools and so on with a diamond film coated thereon have been found to be superior in cutting performance and durability over cutting tools made of superhard alloy. As a result, such diamond film-coated cutting tools capable of cutting for longer periods of time and having longer total cutting tool lives have been sought.
However, unlike conventional cutting tools made of sintered diamond, the cutting tools coated with the diamond film present the problem of durability because flaking may be caused at the cutting parts thereof (i.e., corner portions thereof) due to fatigue.
In addition to the cutting tools, wear-resistant mechanical parts coated with the diamond film likewise have problems with flaking.
As a result of the observation of scars resulting from the flaking of the diamond film from the coated cutting tools, it has been found that in many cases the flaking has occurred at the flank of the cutting tool in the vicinity of the contact point between the cutting part and the cutting work and at the land thereof, not at the contact point.
It is to be noted that the cutting part of the cutting tool so referred to herein means its face, its flank, and a cutting edge intersected by the face and the flank thereof. The corner portions of the cutting tool are subjected to honing, as will be described hereinabove, because ceramics are employed as a substrate for the cutting tools.
In order to solve the problem with flaking as described hereinabove, it is necessary to improve the strength by thickening the film thickness of the diamond film at both the flank and the land of the cutting part as thick as at the face thereof.
To this end, various attempts have been made to coat the diamond film of high quality on a surface of the coating member in a uniform film thickness.
For instance, in a conventional method for coating a diamond film on a body, a body 1 to be coated with a diamond film is placed on a top of disc-shaped or column-shaped supporting member 9 made of h-BN or SiO.sub.2 and having a diameter of 2-4 cm, as shown in FIG. 21. As shown in the drawing, a top surface of the supporting member on which the diamond film-coating body is placed is so much larger than a bottom surface of the diamond film-coating body to be coated with the diamond film that, when the diamond film is formed (for example, by means of the microwave plasma CVD method,) plasma spreads over the entire surface of the supporting member on which the diamond film-coating body is placed, thereby creating a large distribution of temperatures and requiring a large magnitude of electric power for charging microwaves. Therefore, a method has been proposed which reduces the surface of the supporting member on which the diamond film-coating body is placed to such an extent that the surface of the supporting member is not smaller than the bottom surface of the body to be coated with the diamond film, as shown in FIG. 22.
As a consequence, plasma spreading over the entire surface of the supporting body can be reduced, thereby decreasing the distribution of temperatures and the requirement for electrical power for charging microwaves. Also, the diamond film can be thickened and the uniformity of the film thickness at the face as indicated by face 8 of cutting part in FIG. 20 can be improved.
However, this method has the difficulty of thickening the diamond film in the vicinity of the contact point in the flank of the cutting tool.
On the other hand, the method using a substrate or a substrate holder, composed of a dielectric body having an exothermic value by microwaves larger at its outer peripheral portion than at its central portion (as described in Japanese Patent Laid-Open Publication (kokai) No. 62(1987)-167,294). This method can improve uniformity in a film thickness of the diamond film.
This process, however, still has the problem of thickening the diamond film in the vicinity of the contact point on the flank of the cutting tool to be coated with the diamond film because the diamond film-coating body is placed on a disc-shaped supporting member.
More recently, methods have been developed which use an accessory member to thereby avoid localization of plasma by the vapor phase method and maintain uniformity in the distribution of temperatures (as described in Japanese Patent Laid-open Publication (kokai) Nos. 2(1990)-188,494 and 2(1990)-188,495). It is to be noted that these methods likewise have the difficulty of making the diamond film thicker on the flank of the diamond film-coating body.
In other words, the conventional methods for the preparation of the diamond film by using various supporting members all have difficulty in thickening the diamond film at the corner portion of the body to be coated with the diamond film.